Magnetic resonance imaging (MRI) is a medical imaging modality that can create pictures of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis, and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonant frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
During an MRI scan, various elements of the MRI system experience mechanical vibrations, such as the coldhead motor or gradient coil (e.g., as a result of pulsing of the gradient coil). Mechanical vibrations of the MRI system may also be caused by external sources such as floor vibrations caused by a nearby elevator or subway. The mechanical vibrations of such sources can cause the mechanical vibration of other elements inside the MRI system, such as a cryostat thermal shield, and induce eddy currents in electrically conductive material in the cryostat (e.g., the vacuum vessel, thermal shield, helium vessel). The induced eddy currents in, for example, the thermal shield, can result in magnetic field distortion, homogeneity degradation and reduce image quality. The higher the main magnetic field is, the higher the induced eddy current will be and hence the higher the magnetic field distortion.
It would be desirable to provide a system and apparatus to passively (e.g., automatically) cancel or reduce the magnetic field distortion caused by eddy currents induced by mechanical vibrations.